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post #11 of 15 Old 08-08-2012
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Re: How much solar panel(s)?

Originally Posted by rackham the red View Post

What type of refrigeration do you have that is supported by solar, are you talking about AC or just the freezer/refrigerator?

Fridge is an Adler Barber Super Cold machine running on 12 volts. Keeps a LARGE fridge cold and makes ICE. Bonzer bit of kit.

I know of no one running AC off solar. A back of a fag packet calculation shows it is impracticable. You need a genny!
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post #12 of 15 Old 08-12-2012
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Re: How much solar panel(s)?

I'm in the middle of the electrical section of Don Casey's Complete Sailboat Maintenance Manual and I'm getting the 12v calculations down pretty well (electrical has ALWAYS given me a tough time), but when it switches to inverters and alternating current systems I'm getting lost. How do you figure power consumption/amp-hours for, say, a tv?
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post #13 of 15 Old 08-12-2012
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Re: How much solar panel(s)?

You don't figure consumption in amp-hours, you figure it in watt-hours or kilowatt-hours.

When you are working with one voltage (i.e. 12VDC or 110VAC) you can use amp-hours to add up all the devices, and then just multiply by the voltage to get watt-hours. Either way works, as long as you convert at some point to match the amp-hours in your battery bank. Sometimes the loads and rating plates are listed in amps, sometimes in watts, so it may be more convenient to use one or the other for the addition, rather than converting a dozen gizmos individually.

For inverters? Same as anything else, volts times amps equals watts. Watts times hours equals watt-hours. Or amps times hours, directly, if you are using the same handfy voltage as your battery bank.

AC power in the US is generally required to be 117VAC plus or minus ten percent, and you'll see it called 110 or 120 but the exact voltage can vary. From an inverter, you'd have to see what the inverter is set for, and most of them also consume some power. So an inverter than is supplying 100 watts at 120VAC, is probably consuming 103-105 watts on the "12"VDC side. Which of course may be 14.4VDC when the engine is running, or 12.1 volts when the battery is low.

You pick whichever number gives you the WORST results, when you have to use one of those voltages.
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post #14 of 15 Old 08-14-2012
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Re: How much solar panel(s)?

Originally Posted by hellosailor View Post
"how much do I need."
You know the CapitolOne commericals? "SHOW ME YOUR WALLET!"

Buy till it hurts, then stop. That usually fits the bill.

And of course, whether you want to spend the bucks for an MPPT controller, which should give you a 10-15% boost (more watts) from your panels.

Yeah, I believe you are right on with your estimate for a MPPT although I can't help running numbers in my mind. I have my 2 new 145 watt DM poly panels from Amazon (320 delivered) installed, today was my first full day with them. The sky was mostly sunny but there were high clouds...Cirrus? that surely had an impact. My controller is a Morningstar Prostar 30, not a MPPT.

The best I could get today was just over 11 amperes with the two panels in parallel. If things were perfect I should see over 16 amperes. Now getting to an MPPT performance. If I got say a 10% improvement, heck, that is only another 1.1 ampere.

Now for a lot of assumptions--- assuming the panels in parallel are outputing 10 amperes at a voltage of say 20 vdc... I don't want to look up the specs, that is an output capacity of 200 watts which is reasonable. Because in my assumed example, the load (battery) is only receiving 14v*10a or 140 watts so the performance loss is 60 watts. With the better controller I expect a 10 betterment so under this condition the load could receive 14v*11 or only 154 watts.

Does this seem logical??? Heck, a good MPPT rated at 40 amperes is going to cost over $400. I need better performance although I am over joyed just have the additional help from the panels. Now if I add another set of panels when Amazon gets them in stock, I should be able to double my current high of 11 but for discussion, I will use only 10 amperes for my base that will be doubled to 20 amperes.

That is a 100% improvement. So from this simple analysis with a lot of assumptions, forget the expensive controller, just get more panels.

Now I am new to solar....... does the above seem logical?


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post #15 of 15 Old 08-14-2012
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Re: How much solar panel(s)?

Foggy, other DM panels say:
145w 0-3%
Cells by GinTech
VMP 18
IMP 8.05
ISC 8.95
VOC 21.6

which means they put out 144 watts at a peak voltage of 18 volts with a maximum amperage of 8 amps. If you are using any kind of regulator, even a pwm regulator, that should be reducing your voltage to perhaps 14 volts. Which would leave you with 14 volts at the same maximum of 8 amps, for 112 watts. Your loss of 32 watts, or about 23% of the power, is being thrown away by your controller and ignoring whatever the internal loses are (often quoted at 2-4% for MPPT controllers) that means upgrading to an MPPT controller could give you a fast 20% more power from the same panels. If your batteries are discharged, and your controller is putting out 12.5 volts instead of 14 volts, your controller is throwing away even MORE power and the MPPT gain is even greater. And that's going to be the case most of the time.

Is that worth $400? Dunno, can you live with what you have, do is that difference enough so that you don't have to buy another panel to keep the batteries topped up? That's a subjective choice to make.

If you can find curves from DMSolar that show amperage output versus voltage output, you can compare to see if this 20% difference (which is based on full light, i.e. the two hours around noon) gets bigger or smaller during the rest of the day.

Of course, with Amazon's generous returns policies, I'd say that IF you can afford an MPPT controller, buy one with the next two panels. Run two with the MPPT, two with the PWM, and use a charge monitor or a little ingenuity to actually compare what each system is giving you. Math is all well and good, but I like to see systems run side by side with realworld comparisons.

If the MPPT kicks ass and gives you better output, keep it! If it doesn't do much, back it goes. When I ran some tests on an MPPT it took me about two hours to call the maker and ask why the funny numbers I was getting, why it wasn't showing typical "3 stage" voltage changes. It turns out, MPPT can be smarter than that, and designed to maximize the power push at the least voltage needed as well. less acid boiling, higher efficiency...they're impressive to see and measure in action.

Worth the extra cost? Dunno. Is a Ferrari a waste of money? Is four wheel drive? Only one way to find out, right? :-)

Last edited by hellosailor; 08-14-2012 at 11:12 PM.
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